1. Field of the Invention
The present invention relates to zoom lens systems, and more particularly to zoom lens systems for use in an optical apparatus, such as microfilm projectors, having a finite conjugate distance.
2. Description of the Related Art
Microfilms are prepared often with images recorded thereon that are positioned vertically or horizontally at random. It is therefore likely that some images are projected as inclined at an angle of 90 degrees. Accordingly, microfilm readers or reader-printers are usually provided with an image rotation prism between the projection lens and the screen, i.e., on the enlargement side of the lens, whereby the image to be projected is suitably rotated for the correction of its position.
With an arrangement wherein the image rotation prism is disposed on the enlargement side of the projection lens, the projection lens must be short in its overall length in view of the space needed for providing the lens.
This requirement is a great limitation to projection lenses of long focal length with a magnification of about X10. The requirement imposes a still greater limitation on zoom lens systems including a magnification of about X10 within the zooming range.
U S. Pat. No. 4,733,951 discloses a zoom lens system which is provided with an image rotation prism similarly on the enlargement side. The disclosed zoom lens system has a zooming range of X6.5 to X14 and is of the type comprising two groups of lenses, i.e., a first lens group having a positive power, and a second lens group having a negative power. The zoom lens system has a telephoto ratio (.infin.TL.sub.L /f.sub.L) of 0.78 when set to the longest focal length with the object point at infinity.